First & foremost, AS can't cancel your SEA-PDX flight if the coupon is on QR ticket stock. They have no control over it until the day before the flight.

If AS was in the practice of cancelling impossible tickets, which from my experience, they are not, the most they could do would be to cancel the SEA-YVR tickets.

I have had, on more than one occasion, flights overlapping. One was exactly your situation. The last leg of our itinerary was SEA-LAX on a through fare. There remaining segment had no value, so I didn’t bother doing anything with it. I booked a SEA-PHX flight that was flying at the same time. I called AS from the N lounge to advise them I wouldn't be on the LAX flight. They thanked me and we flew off to Phoenix.

I wouldn't have any qualms about booking the SEA-YVR flights. IRROPS in SEA at that time is a real possibility. With an AS ticket, you have a lot more flexibility to cancel, or make changes with a waiver.

If you go the AC route, make sure you purchase a fare that can be canceled to a credit voucher, so you don't lose the value of the tickets should you not be able to fly as scheduled. I would be more comfortable dealing with AS than AC.

AC does handle credit vouchers well, though. You get two emails, one for the voucher and another with a PIN. The funds never expire, can be used by anyone & the currency of the new ticket doesn't have to be the same as the currency of the voucher. When booking online, the voucher is converted and applied. If you have a voucher in USD from SEA-YVR and later wanted to use the funds on YVR-PDX in CAD, that isn't an issue.

My most bizarre experience was last February. I had a QR award, CPT-DOH-ATL/stopover/ATL-SEA-PHX. I flew the CPT-DOH-ATL portion. My plans changed and I didn't need the ATL onward. I called AS to cancel & was advised they couldn't do it. I had booked an AA ticket SEA-SCL with 2 AS plated segments, the first segment was departing SEA on the same day as the ATL-SEA-PHX.

I decided I would take the red-eye SEA-ATL flight the night before, return on the ATL-SEA award flight in the morning (same aircraft) and be back in time for my SEA-DFW despite also having a SEA-PHX award segment. Seattle had a 20 minute snow event that night and chaos ensued. I boarded the flight to ATL. Dicing was a 2+ hour wait. Then the plane went MX with the APU failing. When the door reopened and maintenance boarded, I got off the plane. They reopened the flight to take me off. I headed to a hotel for the night.

AS refunded the flight to OFP since the flight went MX during WX. I was marked as a no-show on the ATL-SEA flight. The downstream SEA-PHX was never cancelled & I flew the AS SEA-DFW segmented plated on the AA itinerary. Nothing was ever flagged as duplicate or impossible. Ultimately, I was a no-show for the SEA-PHX flight, too.

In conclusion, book the AS SEA-YVR itinerary and don't be concerned about it.

James